Alert crabs outwit predatory birds

Sydney, April 19 (IANS) Fiddler crabs would all end up as dinner for seagulls but for their sophisticated threat detection system.

Researchers at the Vision Centre of the Australian National University have figured out how crabs distinguish between threats and non-threats by looking for visual cues.

“These crabs can’t distinguish shapes and must constantly compete with birds that are bigger and have sharper sight,” says Jan Hemmi from the Vision Centre at the Australian National University, the journal Proceedings of The Royal Society B reports.

“They have to run to their burrows to escape passing birds as often as every two or three minutes,” Hemmi adds, according to a Vision Centre statement.

The world’s first research into how crabs see birds is throwing new light on the visual cues that small animals use to recognise the danger signals given off by potential predators.

“We used dummy predators in the form of styrofoam balls and the crabs were scared witless of them - that’s how bad their eyesight is,” Hemmi says.

“So instead of focusing on the shape of the bird, which they can’t discern, they look at its height, the speed of its movements when it swoops, and how it flies,” Hemmi said.

“As soon as the predator flaps its wings, the flickering light is detected by one ommatidium (photoreceptor or vision cell) of the crab’s eyes, which is akin to one pixel on a computer screen.”

The size and relative speed of the bird detected by the crabs’ photoreceptors tells it whether the bird is getting closer - or simply passing over. A flapping bird might indicate an approaching threat - while a soaring bird might be just passing.

“Crabs’ eyes are two centimetres above the ground,” he says. “But they have also become very selective in discriminating between dangerous and non-dangerous situations.”